Summary of studies using the transgenic rats: 1. Based on previous studies in the literature we had expected to find significant microglial activation in the brains of the HIV transgenic rats which would increase with age. In order to develop an imaging biomarker we decided to use 18F-DPA714 PET imaging at three different age groups: young, middle aged and old (3, 9 and 16 month-old). We compared Tg to age matched controls at each of those ages. We used the quinolinic acid animal model as a positive control. Contrary to our expectations, we observed only slightly higher mean SUVc (SUV corrected for weight) values in the Tg rats compared to age-matched WT rats, which did not reach statistical significance in any region or age group. Similarly, immunofluorescent staining for Iba1 in the Tg rat brains at different ages showed no significant differences when compared to age matched controls. We further confirmed our findings by measuring cytokine levels in the brain lysates of Tg and age-matched controls and found them to be similar. 2. We further performed a detailed immunohistochemical evaluation of the Tg rat brains at different ages, and compared to age-matched WT rat brains. The most significant finding in the Tg rat brains was progressive loss of GFAP staining, suggesting astrocytic dysfunction/damage, significantly worsening with age in the striatum and corpus callosum. Besides decreased conspicuity of the astrocytic cell bodies, there was definite loss/attenuation of astroctytic processes in the Tg rats. In comparison, the microglia appeared to be slightly activated early-on (1-3 months of age), but this decreased as the animals got older. 3. Despite the motor and behavioral deficits we saw in the Tg rat compared to WT rats, the longitudinal and cross-sectional differences by 18F-FDG -PET did not reach significance between the two cohorts. These results were further confirmed by the lack of significant difference in glucose metabolism as measured by 14C-DG-autoradiography performed on the same animals after the last FDG imaging sessions. We observed however significantly decreased D2/3 receptors in the adult Tg rat brains compared to age-matched controls, in both the dorsal striatum (DS) and ventral striatum (VS), using 18F-fallypride PET imaging. In younger animals, the decrease in D2/3 receptors was only significant in the DS, suggesting that there was a continuous trend of neurological/dopaminergic damage that worsens with age. The combination of decreased TH that we saw on immunofluorescence (early step in dopamine synthesis) and decreased post synaptic receptor density (D2/3) seen on PET supports the notion of dopaminergic neuronal dysfunction/loss in this animal model. We concluded that 18F-fallypride can detect dopaminergic system dysfunction in the Tg rats and may be a reasonable imaging biomarker to evaluate neuroprotective approaches. 4. We are now evaluating the role of oxidative stress in the neuropathology of the tg rat. Summary of studies using the SIV infected monkeys: 1. We hypothesized that 11C-DASB binding will decrease in the acute phase of SIV infection (with respect to pre-inoculation scans) and will potentially reverse with ART treatment. We are using rhesus macaques infected with the neurovirulent SIV strain, SIVsm804E. We are performing PET imaging targeting SERT using 11C-DASB in monkeys before and after inoculation, in order to detect serotonergic system status under the effect of SIV infection. We found that the serotonin transporter (SERT) levels/expression increased in most animals (6 out of 7) who were infected with SIV when we compared the last time point imaging (11C-DASB PET) to the preinoculation scans. We are investigating a potential epigenetic effect of the virus on SERT expression at this point. 2. We are also investigating the evolution of neuroinflammatory changes in the SIV-infected monkeys using peripheral viremia and disease control (simulating optimally treated HIV+ patients) as covariants. Towards this goal, we are using 18F-DPA-714 as PET imaging biomarker of microglial activation (neuroinflammation), in vivo, in the SIV-infected monkeys. We are imaging pre-inoculation as well as post-inoculation, before and after treatment. Unlike what we expected, we did not find increased translocator protein expression using 18F-DPA714 PET imaging in the monkeys (5 out 5 showing decreased binding after inoculation rather than increased binding). We believe this correlates to the severity of CNS involvement as measured by CSF viral load. We are now continuing the imaging study after treatment and interruption of treatment.